In Skylanders: Lost Islands, Radiance is a source of energy used by Skylanders during adventures. It's obtained by harvesting plants in great quantities, and is stored in bottles. It is unknown if the Radiance is exported from the Radiant Isles or it's also found in the Lost Islands.

Radiance Rush

Collecting 10 Radiance rapidly will give you a Radiance Rush, which doubles the amount of Radiance obtained. When you have a Radiance Rush, all Radiance will change colors from white to red. A Radiance Rush only lasts for a limited amount of time.

Trivia

During the development of the 3DS version of Spyro's Adventure, Radiance was known as Brightstone.[1]